An odd combination of attributes led McNeill Alexander to uncover the secret of dinosaur locomotion.

Armed with only a measuring tape and his teenage children, Alexander used the soggy sands of Snettisham in north Norfolk to show that these extinct giants were really fairly mobile.Contrary to popular belief, dinosaurs did not always lumber about like sluggish behemoths, he says. Instead, they could occasionally nip about in a fairly sharpish way.

It's an interesting conclusion, arrived at by answering a short, simple question: Does boggy ground affect the length of one's stride?

It sounds trivial, but in fact the answer is important, for Alexander, a professor at Leeds University's pure and applied biology department, had previously calculated how fast dinosaurs might have run from estimates of the lengths of their legs (gleaned from fossils) and from estimates of their strides (based on footprints preserved in ancint sediments).

But his equations had assumed that the dinosaurs had been running on a hard surface. In fact, they must have been moving over boggy ground for their footprints to have been fossilized.

So does sogginess affect length of stride, he wondered?

To find out, he planned a simple experiment. He took his children Jane and Gordon on Christmas Eve to Snettisham, which has long beaches with hard and soft sands. They then ran about for hours, while he measured their footprints.

From these he found that speed is reduced in muddy ground, but length of stride is unaffected. His original calculations therefore stood: Dinosaurs were capable of moving at speed.

It was a typically idiosyncratic experiment, carried out with aplomb and absurd simplicity by a man who thinks hard before embarking on a piece of research. Not for him the massive computing program. The simple, neat experiment is best, he believes.

Bizarre is an apt description for work that has included filming ferrets galloping, that uses amputated feet to test running shoes, and which involves studies of camels to uncover the secrets of human motion.

"Most other scientists study how humans move from a medical point of view," he says. "I do it through studies of all animals, from sea anemones to rhinos. I treat humans as members of the animal kingdom and that provides a special insight.

"There is a remarkable uniformity in the way animals move. If you know at what speed a horse begins to gallop, you can calculate at what speed rhinos or ferrets start to gallop."

By using hydraulic ramps to calculate stresses on amputated feet left over after operations and from other means, he found two mighty springs that control humans' gait. One is the Achilles tendon, and the other is the sheet of tendons in the arch of the foot. These store energy as our legs move down and release energy as they lift, preventing energy waste - a trick that running-shoe makers should note.